On previous iPhone devices holding down the lock button brought up the "Slide to Turn Off" screen. From the keynote Apple said that holding down the side button brings up Siri now. How do I turn off iPhone X?
I can see three ways to turn off an iPhone X.
Press and hold any Volume key + the side lock key, together. Press and hold these keys for a few seconds and the “slide to power off” option will appear on the screen.
Go to Settings >> General >> Shut Down.
Using Assistive Touch. Just go to the “AssistiveTouch” menu, then select “Device." Tap and hold the Lock Screen icon. Within seconds this will bring up the “slide to power off” option on your screen.
Source - Mobipicker
Hold down the Volume Up or Down and Side button to Power Down
initiating power off/Emergency SOS by pressing and holding either volume button and the side button simultaneously for 2 seconds